Local forest die-offs could have global consequences, study shows

Forest die-offs -- whether from drought, deforestation or invasive insects -- have far-reaching ramifications that could affect plants across the country and around the globe similar to other more well-known environmental phenomenon like El Nino.

Trees have been dying off around the world for years, in eastern Oregon and Washington locally, but much of the impact was assumed to be local. A new study from researchers at the University of Washington and the University of Arizona shows that the ramifications of a massive tree die-off can ricochet around the world with some unexpected consequences.

Those findings were released Wednesday in a study published in the journal PLOS ONE. Abigail Swann, an assistant professor of atmospheric sciences and of biology at the University of Washington and co-author of the study, said little investigation had been done on the interaction of plant mortality and climate.

"People have thought about how forest loss matters for an ecosystem, and maybe for local temperatures, but they haven't thought about how that interacts with the global climate," Swann said. "We are only starting to think about these larger-scale implications."

The team of researchers, which included professors from around the world, focused on two areas at risk of major forest die-offs: the western United States, including Oregon, and the Amazon rainforest. In their first model, which they concede looked at extreme examples, the team created a scenario in which most of the trees in the west were removed.

The model showed some expected ramifications nearby -- the air became cooler and drier with fewer trees to absorb sunlight -- but there were also effects as far away as the southern United States and Siberia, where the air became cooler and forest growth slowed. The model saw a benefit for South America, where forests would likely see increased growth due to increased moisture.

In their second model, researchers removed vast swaths of the Amazon rainforest to see what the global impacts would be. The results showed a positive impact for plants in the southeastern U.S. and similarly in the eastern regions of South America where precipitation would likely increase during the summer.

The scenario modeled in the experiment -- wherein forests are wiped out region-wide -- is not likely to come to fruition, said Dave Breshears, a professor of natural resources at the University of Arizona and co-author of the study. Still, the risk to forests is stark.

"We are seeing die-off events all around the world, on every continent that has trees," Breshears said. "Nowhere is invulnerable."

In the west -- specifically in Oregon, Washington and California, forests have been battered by extended droughts and bark beetles, which can hasten the demise of forests weakened by dry winters.

"The vulnerability of forests is much higher than we expected even a few years ago," Breshears said. "Some studies have shown that we could be looking at a 50 percent loss of our coniferous forests by the year 2100."

As scientists continue to grapple with the complicated effects of a changing climate, this type of research will be of increasing importance, said Tim Kratz, program director at the National Science Foundation, which funded the study.

"This study shows that local events like forest die-offs in one part of the globe influence climate and ecology in other, often distant locations," he said. "Unraveling these far-reaching effects is critical to understanding how nature works at continental to global scales."